This study was aimed to evaluate the single and combined effects of acidifier and probiotic on growth performance, digestive enzymes activities and intestinal histomorphology of Salmo trutta caspius. The juvenile fish with mean body weight 15±3 g were divided into 8 different treatments (in triplicates), including control, 5, 10 and 15 g sodium diformate kg-1 diet, respectively as T1, T2 and T3. Treatments namely T4, T5, T6 and T7 were received diets containing 0.2 g kg-1 commercial probiotic Bio-Aqua® in combination with 0, 5, 10 and 15 g sodium diformate kg-1 diet for 60 days, respectively. The results showed that T2 and T3 fish growth performance were improved significantly (p<0.05), following 30 days after administration, while T1 did not show the same pattern over the 60 days (p<0.05). The single probiotic treatment did not induce significant improvements in fish growth rate, digestive enzymes activities and intestinal morphometry though the combined treatments have been showed an intermediate level of improvement. The higher levels of chymotrypsin and trypsin have been observed at day 30 and the higher activities of lipase, protease and amylase could be seen at day 60 in the most acidifier treatments (p<0.05). The villi height and the thickness of epithelium have been reduced (p<0.05) because of single acidifier while the combined treatments led to either significant increase (p<0.05) or no change compared with corresponding single treatment. The addition of 1.0 g sodium diformate kg-1 diet can improve the fish growth rate in long-term by changing digestive enzymes activities, and combined treatments of probiotic and acidifier are mostly revealed antagonist effects.